Breathing LED on a Raspberry Pi

README.md

Raspberry Pi Breathing LED

This is a demonstration of using Ruby and the WiringPi Gem to pulse an LED connected to a GPIO pin in a manner similar to the sleep indicator on a MacBook.

A video of the effect can be found here: http://www.youtube.com/watch?v=NCUMXK7qf-c

breathing.rb

#!/usr/bin/env ruby
# Raspberry Pi Breathing LED 
#
# Based on:
#  "Breathing sleep LED, like on a Mac." by Jeremy Saglimbeni 2011
#  http://thecustomgeek.com/2011/06/17/breathing-sleep-led/

require 'wiringpi'
io = WiringPi::GPIO.new(WPI_MODE_GPIO)
pin = 18
range = (15..255).to_a
range += range.reverse

def sleep_func(i)
  sleep(0.004) if (i > 150)
  sleep(0.005) if (i > 125) &&  (i < 151)
  sleep(0.007) if (i > 100) &&  (i < 126)
  sleep(0.010) if (i > 75)  &&  (i < 101)
  sleep(0.014) if (i > 50)  &&  (i < 76)
  sleep(0.018) if (i > 25)  &&  (i < 51)
  sleep(0.019) if (i > 1)   &&  (i < 26)
end

loop do 
  range.each do |i|
    io.pwmWrite pin, ((i/255.0) * 1000).to_i
    sleep_func i
  end

  sleep 1
end

Hi. I'm wondering with your setup, what actually does the modulation? I have my Cobbler and RPi set up in the exact way yours is in the video, but I'm not getting the breathing effect. Is the pin altering the current or the voltage? My understanding is that 3.3 volts is connected to the same side of the resistor as pin 18. How does the pin change the current (or the voltage) going into the LED?

Thanks
Alistair